Question: The scale on a map is 9cm : 6km. If the distance between two cities is 12km, how far apart in cm are the two cities on the map?
Answer: The scale means that every 9cm on the map corresponds to an actual distance of 6km. An actual distance of 12km is the same as 2 $\cdot$ 6km. The distance between the two cities on the map is 2 $\cdot$ 9cm, or 18cm.